1. Field of the Invention
The invention relates to high performance low acyl and partially deacylated gellan gum compositions having increased molecular weight and increased gel strength. The invention also relates to processes for producing high performance low acyl and partially deacylated gellan gums having high clarity without using the conventional filtration process. The invention further relates to industrial products comprising high performance low acyl and partially deacylated gellan gums for food and non-food applications.
2. Description of Related Art
Polysaccharides, which are also referred to as gums, are primarily used to thicken or gel aqueous solutions. Polysaccharides that are produced by microorganisms of the genus Sphingomonas are also referred to as sphingans. Gums are frequently classified into two groups: thickeners and gelling agents. Typical thickeners include starches, guar gum, carboxymethylcellulose, alginate, methylcellulose, xanthan gum, gum karaya, and gum tragacanth. Common gelling agents include gellan gum, gelatin, starch, alginate, pectin, carrageenan, agar, and methylcellulose.
Gelling agents are used in the food industry in a variety of applications, including confectionery jellies, jams, dessert gels, icings, dairy products, beverages, and the like. Additionally, gelling agents may be used as components of microbiological media. Gelling agents differ in the conditions under which they may be used and in the texture of the gels they form. These distinctive properties of gels have led to the widespread use of certain gelling agents in particular products (e.g., starch in confectionery jellies; gelatin in dessert gels; agar in icings; and alginate in pimento strips).
One particularly useful gelling agent is gellan gum, which is a capsular polysaccharide produced by the bacterium Sphingomonas elodea, ATCC 31461, and strains derived from this species. The constituent sugars of gellan gum are glucose, glucuronic acid and rhamnose in the molar ratio of 2:1:1. These are linked together to give a primary structure comprising a linear tetrasaccharide repeat unit (O'Neill M. A., et al., Structure of the acidic extracellular gelling polysaccharide produced by Pseudomonas elodea, Carbohydrate Res., 124(1):123-133 (1983); Jansson, P. E., et al., Structural studies of gellan gum, an extracellular polysaccharide elaborated by Pseudomonas elodea, Carbohydrate Res., 124(1):135-139 (1983)). In the native or high acyl (“HA”) form, two acyl substituents, acetate and glycerate, are present. Both substituents are located on the same glucose residue and, on average, there is one glycerate per repeat unit and one acetate per every two repeat units. In the low acyl (“LA”) form, most of the acyl groups have been removed to produce a linear repeat unit substantially lacking such groups. X-ray diffraction analysis shows that gellan gum exists as a three-fold, left-handed, parallel double helix (Chandraskaran, R., et al., The crystal structure of gellan, Carbohydrate Res., 175(11):1-15 (1988); Chandraskaran, R., et al., Cation interactions in gellan: An x-ray study of the potassium salt, Carbohydrate Res., 181:23-40 (1988)).
LA gellan gums form gels when cooled in the presence of gel-promoting cations, preferably divalent cations, such as calcium and magnesium. The gels formed are firm and brittle. HA gellan gums do not require the presence of cations for gel formation and the gels formed have structural and rheological characteristics which are significantly affected by the acyl substituents. Thus the properties of HA gellan gums differ significantly from those of LA gellan gums. HA gellan gum gels are typically soft and flexible and lack thermal hysteresis.
Commercially, gellan gum is formed by inoculating a fermentation medium under aerobic conditions with Sphingomonas elodea bacteria. The fermentation medium contains a carbon source, phosphate, organic and inorganic nitrogen sources, and appropriate trace elements. The fermentation is conducted under sterile conditions with strict control of aeration, agitation, temperature, and pH. Upon completion of the fermentation, the viscous broth is pasteurized to kill viable cells prior to recovery of the gum.
Gellan gum displays different characteristics depending upon the method of recovery from the fermentation broth. Direct recovery from the fermentation broth yields gellan in its native or high-acyl form, which is modified by S. elodea with acetyl and glyceryl substituents on one glucose residue. Isolation of gellan in this native or high-acyl form yields a soft, flexible, elastic gel. Gellan may be deacylated to provide gellan in its low acyl form. Isolation of gellan in this low acyl form yields a hard, firm, brittle gel. Blends of native and low acyl gellan produce gels of intermediate texture.
Currently, gellan gum is deacylated by treating the fermentation broth containing the gellan gum with strong alkali at high temperature. This process removes acyl substituents from the gellan and lyses the S. elodea cells. Solids and cell debris are then removed by acid treatment (to neutralize/acidify the fermentation broth) and filtration to yield a high clarity, low acyl gellan gum. However, this method also results in a gum molecular weight substantially lower than that produced by the native organism, due to depolymerization as well as deacylation. The current commercial method for gellan gum recovery produces gels with a maximum 0.2% gellan gum curdmeter gel strength of about 290 g/cm2 (equivalent to a 0.1% gellan gum curdmeter gel strength of about 113 g/cm2).